Coating media are known in which resins which contain hydroxyl groups are crosslinked with polyisocyanates. Thus EP-A-0 541 604 describes coating media based on polycondensation and polyaddition products which contain hydroxyl groups and which are crosslinked with polyisocyanates, wherein the binder vehicle which contains hydroxyl groups is composed of at least one polyester and/or one alkyd resin and at least one polyacrylate resin, wherein the polyacrylate resin is produced, at least in part, in the presence of the polyester which is contained as a proportion. Customary organometallic compounds, such as dibutyltin laurate, are employed as catalysts here. Substances of this type are not toxicologically harmless, and moreover often give rise to yellowing. EP-A-0 036 975 describes a two-component lacquer which is composed of a polyester polyol which is low in aromatics, and optionally of a hydroxy-functional polyacrylate resin, which are crosslinked with polyisocyanates. Zinc octoate, which in combination with UV absorbers often results in lacquers which exhibit pronounced yellowing, is used as the catalyst here. EP-A-0 206 072 describes polyacrylate copolymers which are synthesised in the presence of polyester polyols. These binder vehicles can be crosslinked with polyisocyanates or melamine resins. Acids, such as paratoluenesulphonic acid for example, are described as catalysts. The use of acids of this type also frequently results in pronounced yellowing in clear lacquers.